The present invention relates to a method for preventing large current flowing out from video signal line driver into video signal line, when video signal line is shorted to ground.
In general, video signal is connected to IC through AC coupling capacitor. A video signal line driver circuit will transmit the video signal at a pre-determined DC level. One of the ways to implement the video signal line driver is shown in FIG. 1.
Referring to FIG. 1, video source 8 is connected to video signal line 10 through AC coupling capacitor 9. A typical video signal line driver circuit comprises transistors 1, 2, 3, 4 and current sources 5 and 6. Current source 5 and transistors 1 and 2 form a level shifter circuit so that voltage level at the base of transistor 3 is 2Vbe above bias voltage 7. Function of transistor 3 and current source 6 is to provide sufficient base current for transistor 4. Transistor 4 is a large NPN to provide sufficient current drive to transmit video signal along video signal line 10 at a pre-determined DC level. The pre-determined DC level for this case is: (bias voltage 7+Veb of transistor 1+Vbe of transistor 2−Vbe of transistor 3−Vbe of transistor 4).
FIG. 2 shows an example of a problem that may exist in video signal line driver shown in FIG. 1. The problem arises as a result of the need to ground unused video signal lines. This is because of unused video signal line is not allowed to be left unconnected or ‘float’ as it will collect noise and affect performance of the video system. However, as a result of this measure, transistor 4 will source out large current when video signal line 9 is shorted to ground terminal 11.
An alternative solution would be to short unused video signal line in video system to ground via an external capacitor. However, this results in additional hardware cost being spent on grounding unused video signal lines.
The present invention is created to solve this problem.